Molecular Orbitals of Ethene

Transcription

1 Molecular Orbitals of Ethene 1

2 Molecular Orbital Analysis of Ethene Dimerisation the reaction is said to be a "symmetry forbidden" interestingly, this reaction is rare and very slow! Molecular Orbitals of 1,-Butadiene

3 Molecular Orbital Analysis of Diels-Alder reaction

4 Acid-Base Chemistry Reading : MT 6 The Acid Base Theory of Brønsted and Lowry In 19, within several months of each other, Johannes Nicolaus Brønsted (Denmark) and Thomas Martin Lowry (England) published essentially the same theory about how acids and bases behave. An acid is a "proton donor." A base is a "proton acceptor." 4

5 Cl + O <===> O + + Cl Cl - this is an acid, because it has a proton available to be transfered. O - this is a base, since it gets the proton that the acid lost. Now, here comes an interesting idea: O + - this is an acid, because it can give a proton. Cl - this is a base, since it has the capacity to receive a proton. Notice that each pair (Cl and Cl as well as O and O + differ by one proton (symbol = + ). These pairs are called conjugate pairs. Cl + NaO ---> O + NaCl stronger acid stronger base Weaker conjugate acid weaker conjugate base N O - ---> O + N weaker weaker stronger stronger conjugate conjugate acid base acid base 5

6 6 O A O A n n 1 ] [ ] ][ [ log 1 1 A O A pk n n a weak pk strong pk a a 0 0 O A O A n n 1 O N O N 4 Amphoteric Compound

7 Oxyacids AOp(O)q A=Si, N, P, As, S, Se, Te, Cl, Br, I p=# of nonhydrogenated oxygen atom + A ----O As atom A is rendered more positive, it becomes easier to break the O- bond Because of enhanced bond polarization. The greater the formal charge on A. the stronger the acid should be. Oxyacids AOp(O)q p=# of nonhydrogenated oxygen atom p=0: p=1: p=: p=: PKa 8-9 very weak PKa 1- weak PKa - ~ - strong PKa -7 very strong A=Si, N, P, As, S, Se, Te, Cl, Br, I 7

8 - O p= 1 0 Empirical Rule (Pauling) PK a 8 5 p 8

9 As successive protons are removed, the PKa increases by ~4 Or 5 each time. PO PO 4 4 PO 4 (.15) (7.0) (1.7) SO SO 4 4 (.0) (.0) 9

10 nx 0.1 M Cl, S,O, PO4,PO4 -,PO4 -,PO4 - Acidity Se > S > O Conjugated bases Se -, S -, O - of larger molecules lower charge density weaker attraction for + 10

11 Acidity N < O < F Conjugated bases N -, O -, F - N - -1/ on each lone pair O - -1/ on each lone pair F - -1/4 on each lone pair Strongest attraction for proton Strongest conjugated base Weakest acid N Lewis Concept A base: an electron-pair donor Species with lone pair type orbitals An acid: an electron-pair acceptor Species with empty non-bonding type orbitals MO theory 11

12 * u * g Lewis acid BeL, MgL 1 u b 1 u b 1 g Ligand Group Orbitals LGOs Lewis acid BL, AlL GaL InL * x * s Chem * 104A, UC, Berkeley y e ' 1 a ' 1 p x p y s B p z b x nb z b s b y '' 1a ' 1e 1 ' 1a 1 LGO LGO LGO1 B 1

13 N (C v : E, C, v ) Lewis base NL, PL, AsL, SbL, BiL z N () (1) () y x The symmetry of s group orbitals: N ev e (p x, p y ) 4a 1 e -1.5 ev e - s + s a 1 s1 - s - s r = A 1 + E a 1 (p z ) a 1 s 1 + s + s C v E C v A z x +y, z A E -1 0 (x,y) a 1 (s) -5.6 ev 1e ev -1.0 ev a 1 x ev O y p z p y p x z LGO 1s Other: OL -, SL -, SeL - F- Cl- Br- I- -.4 ev s Lewis Base OL, SL, SeL, TeL O O 1

14 Type of non-protic acid-base reactions: 1. Adduct formation: acid-base react to form a bond and produce a single molecule.. Displacement: one base (or acid) displace another. Double displacement (Metathesis): interchange of acids and bases 14

15 * x * s Chem * 104A, UC, Berkeley y e ' 1 a ' 1 p x p y s B p z b x nb z b s LUMO b y '' 1a ' 1e 1 ' 1a 1 LGO LGO LGO1 B N (C v : E, C, v ) z N () (1) () y x The symmetry of s group orbitals: r = A 1 + E N ev e (p x, p y ) 4a 1 e OMO -1.5 ev e - s + s a 1 s1 - s - s a 1 (p z ) a 1 s 1 + s + s C v E C v A z x +y, z A E -1 0 (x,y) a 1 (s) -5.6 ev 1e ev -1.0 ev a 1 15

16 Adduct formation AlCl B SO Cl CO AlCl 4 B C O Et O O S OEt Displacement: one base (or acid) displace another A B B' A B' B F B SEt Et O F B OEt SEt Double displacement (Metathesis): interchange of acids and bases A B A' B' A B' A' B Me Si Cl F B F Me Si F F B Cl 16

17 Relative strength of acid/base Reference acid Base strength +,Mg +, Sc + F Cl Br I g + F Cl Br I ard and Soft Acids and Bases (SAB) ard (class a): having contracted (tightly held) frontier orbitals Not readily polarized Soft (Class b): aving diffuse frontier orbitals Readily polarized 17

18 ave vacant orbitals held in tight ave vacant orbitals, but larger radius ave lone pairs in tightly held orbitals ave lone pairs in larger orbitals 18

19 Table 1 ard and Soft Acids and Bases ard Bases Soft Bases Borderline Bases O O - F - R S RS RS - ArN C 5 5 N AcO - SO - 4 Cl - I - R P (RO) P N - Br - CO - NO RO CN - RCN CO NO - RO - R O N C 4 C 6 6 RN - R - ard Acids Soft Acids Borderline Acids + Li + Na + Cu + Ag + Pd + Fe + Co + Cu + K + Mg + Ca + Pt + g + B Zn + Sn + Sb + Al + Cr + Fe + GaCl I Br Bi + BMe SO BF B(OR) AlMe C carbenes R C + NO + Ga AlCl Al SO C RCO + CO X (hydrogen-bonding molecules) ard Bases (nucleophiles) Soft Bases (nucleophiles) ard Acids (electrophiles) Soft Acids (electrophiles) Donor atoms have high electronegativity (low OMO) and low polarizability and are hard to oxidize. They hold their valence electrons tightly. Donor atoms have low electronegativity (high OMO) and high polarizability and are easy to oxidize. They hold their valence electrons loosely. Possess small acceptor atoms, have high positive charge and do not contain unshared electron pairs in their valence shells. They have low polarizability and high electronegativity (high LUMO). Possess large acceptor atoms, have low positive charge and contain unshared pairs of electrons (p or d) in their valence shells. They have high polarizability and low electronegativity (low LUMO) n.b The SAB principle is not a theory but a statement of experimental facts. Pearson, R.G, Songstad, J.Amer.Chem.Soc., 1967, 89,

20 Pearson s Principle: ard acids prefer to bind to hard bases And soft acids prefer to bind to soft bases. Keq C g B C gb The simplest hard acid is the proton and methyl mercury cation is the simplest soft acid. Keq small B hard base Keq large B soft base I Br SCN Cl SO S O N S F PPh O CN 0

21 Li-I +Ag-F -F +Ag-I =-17kcal/mol gf +BeI gi +BeF =-95kcal/mol Absolute ardness IE EA ( E LUMO E OMO ) ard: large OMO-LUMO gap Soft: small OMO-LUMO gap, ease of mixing ground/excited states, electron Density redistributed (polarized). 1

22 Strong polar bond! Strong covalent bond! Empirical approach to determine enthalpy of adduct formation A + B AB E A E B C A C B E: susceptibility to undergo electrostatic interaction C: tendency to form covalent bonds

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24 SO Me N Me N SO ( ) 9.kcal / mol Experimental value: -9.6 kcal/mol 4